1. Technical Field
The invention is related to plasma-enhanced semiconductor wafer processing apparatus such as plasma reactors for etching or chemical vapor deposition, having an electrostatic chuck for holding the wafer in place, and in particular to a method of de-chucking or removing the wafer from the chuck with a minimum of force.
2. Background Art
When using an electrostatic chuck to hold a wafer in a plasma reactor chamber, one of the difficulties to overcome is that the wafer adheres or "sticks" to the chuck at the end of the process when the wafer is to be removed from the chamber. The severity of this sticking can range from breaking the wafer while attempting to lift it off of the electrostatic chuck to misplacing the wafer on the wafer-handling robot's blade when the wafer is being unloaded from the reactor chamber. The sticking is attributed to static charge buildup on the surface of the electrostatic chuck's dielectric layer facing the wafer.
As disclosed in related U.S. patent application referenced above, it has been determined that there exists a particular voltage value that, if applied to the electrostatic chuck electrode, can compensate for the electrostatic field that causes the wafer sticking, so that the wafer is easily de-chucked with virtually no sticking force to overcome. However, the methods disclosed for determining the optimum dechucking voltage involve sophisticated measurement of a very short electrical pulse that occurs as the wafer is lowered onto the chuck. A goal of the present invention is to determine the optimum dechucking voltage without requiring measurements of electrical pulses.